1. Field of the Invention
Synthetic resinous pipes are generally manufactured with the method of extrusion molding and have for the most part a definite external diameter. In laying such pipes, varieties of connecting means are applied depending on the requirements of the connecting parts used therein. There are instances, of course, when a particular kind of or a special-made pipe joint is employed. However, in ordinary circumstances, a spigot pipe is inserted into a socket part of the other pipe which is secondarily formed at its one end, by taking advantage of the thermoplasticity of synthetic resinous material. As it is, since a socket part must be of larger diameter as compared with the main body of the pipe, accordingly an inclined part is to be inevitably produced at the interior of the socket part, and a space is left in the interior of this inclined part after the spigot pipe has been inserted therein. This space is liable to bring about the accumulation of inclusions or impurities present in the fluid when it is of low pressure such as in the sewer system use, and be accompanied by a disagreeable increase in the flow resistance of the fluid. While, in case of a high pressure fluid such as in a water supply system, turbulence is produced in it, causing water hammer, or the like, and exerting a bad influence on the pipe line. What is worse, at the very time of connecting synthetic resinous pipes, the ignorance of the exact insertion length of the spigot pipe before the connecting operation sometimes leads to damage of the spigot pipe end, or results in the breakage of the forcibly pushed socket part under the heavy load applied to the inclined surface thereof. In order to correct these flaws and to predetermine the insertion length of the spigot pipe, the provision in the interior of the socket part of some kind of a corbel piece is necessitated. This consideration has resulted in the provision in the interior of the socket part with a so-called filler ring which serves to plug up the aforenoted space and at the same time regulate the position of the tip of the spigot pipe. This proposal, in fact, has been already applied in practice. In the case of using only such an arrangement, however, the pipe to be formed which is inserted was not always located in the proper position, or, if it was properly located, the filler ring could not be held on the socket part for a long period of time and was readily broken away therefrom by impact and shock applied to the filler ring.
2. Description of the Prior Art
The arranging means for such a type of filler ring with which we had been working heretofore were the following: a means of arranging the filler ring on the interior of the socket part through the aid of some adhesives after the formation of the socket part, or a means of arranging on the socket part of the filler ring fixed in advance on the core mold which is to form the socket part, synchronously with the formation thereof. Yet, the one with which the filler ring is set after the formation of the socket part can be applied only to the special case of large diameter synthetic resinous pipes and is not available for the socket part which is made in a mass production system. Also, the thus planted filler ring has a weak point so as to be readily moved out of place by an insignificant shock. For such problems, there is popularly utilized the other means with which the filler ring is firstly fixed on the core mold and is then applied to the socket part concurrently with the formation thereof. Nevertheless, since these setting means were no more than simple ones wherein the filler ring had only to be arranged on a prescribed position along the external circumference of the core mold called a "tamper mold", the center of the filler ring sometimes did not coincide with that of the core mold, if there was an irregularity at the circumferential edge of the filler ring. As a result, it often occurred that the filler ring which was planted in the socket part was not concentric with the socket part, and more, there was also the fault that a gap was produced between the filler ring and the inner surface of the socket part, if the filler ring was not perfectly round in regard to its forming accuracy, subsequent to which the filler ring becomes inoperative.
Be that as it may, the extrusion-molded pipe in general is characterized by a fixed size of its external diameter, but it has a flaw of fluctuating in a certain range with respect to its internal diameter. Accordingly, it is common knowledge that the core mold which is used for the secondary molding of the extrusion-molded pipe is provided with a guide section for the sake of the external diameter corresponding to the diameter of the pipe of minimum internal diameter, with the view to the easy insertion into any pipe in the above-mentioned range. In the case of the previously stated core mold for use in loading the filler ring, the external diameter of its guide section was nearly the same as the minimum internal diameter of the inserted pipe. In the case of loading the filler ring, if it was smaller as compared with the internal diameter of the pipe and consequently was in such a manner as to protrude from the inner wall of the pipe, there occurred the aforementioned sundry troubles so that it required that the internal diameter of the filler ring should have been equal to, or a little larger than, the internal diameter of the pipe of a maximum internal diameter. This is the reason why the dimensioning of each diameter should be in accordance with the following order and relation: EQU Max. ID of the filler ring &gt; Min. ID of the filler ring .gtoreq. EQU Max. ID of the pipe &gt; Min. ID of the pipe .gtoreq. OD of the core mold,
(wherein Max. = maximum, ID = inner diameter, Min. = minimum, and OD = outer diameter)
Under such conditions, there was an advantage that the operation of manually loading the filler ring was easy because of the clearance between the filler ring and the core mold, granting that the filler ring was somewhat deficient in roundness. At the same time, this clearance formed are eccentricity between the filler ring and the core mold, entailing the debasement of the retention of the filler ring, and obstructed the insertion of the pipe because the end of the pipe which was to be formed got caught at the time of its insertion. On the other hand, though the curtailment of the clearance made the eccentricity proportionally less, it became inconvenient to set up the filler ring on the core mold. If also the filler ring might be simply arranged on the interior of the socket part, it was only hung on the surface contacted with the socket part, but was not securely held thereon. As a result, on removing the core mold, the filler ring could not be set in the socket part as the same remained on the core mold, or, if set, it was not tightly secured thereon, and, as a natural consequence, there often occurred the separation of the filler ring out of the socket part on account of the contraction of the synthetic resinous material after the pipe formation or by the vibration and impact during their transportation.